Brine tolerant polymer for oil recovery applications

ABSTRACT

A beta-alanine-type branched partially hydrolyzed polyacrylamide is added to an aqueous injection fluid to increase the viscosity of the fluid. The polymer resists plugging of the wellbore face and/or matrix pores and is brine tolerant when injected into a subterranean hydrocarbon-bearing formation.

BACKGROUND OF THE INVENTION

Technical Field

The invention relates to a brine tolerant polymer composition and aprocess for producing and using the same and more particularly to apolymer composition used as a viscosifying agent, which is relativelyinsensitive to saline connate water in a subterranean oil-bearingformation.

Description of Related Art

Polymers are commonly used in oil recovery applications as viscosifyingagents for fluids injected into subterranean oil-bearing formations.Applications for fluids having polymer enhanced viscosity include oildisplacement in water flooding, mobility control in surfactant flooding,and permeability reduction of thief zones.

One method of increasing the viscosity of a fluid is to add extremelyhigh molecular weight polymers to the fluid. A problem concomitant tothe use of high molecular weight polymers is that the polymers oftenfilter out the injected fluid onto the wellbore face or in the matrixpores, causing undesirable face plugging and permeability reduction. Anadditional problem is that high molecular weight polymers are less brinetolerant than lower molecular weight polymers, i.e., high molecularweight polymers ball up in brine, diminishing the viscosity of thepolymer-containing fluid.

The art recognizes brine tolerant polymers for oil recoveryapplications. For example, U.S. Pat. No. 3,880,764 to Donham describes adrilling fluid composition containing a polymer and additives whichrender the polymer relatively brine tolerant. However, an unfilled needexists for other brine tolerant compositions useful in oil displacementflooding and other above-described applications.

SUMMARY OF THE INVENTION

The present invention provides a brine tolerant low molecular weightpolymer composition which effectively increases the viscosity of a fluidcontaining the polymer. The polymer composition is a partiallyhydrolyzed polyacrylamide (PHPA) wherein one or more of the amide groupsof the PHPA have a beta-alanine-type constituent added thereto.

The composition of the present invention may be produced by contacting abase such as sodium hydroxide with a mixture containing unhydrolyzedpolyacrylamide and acrylamide monomer. The base reacts with the monomerto form beta-alanine-type constituents and hydrolyzes a portion of theamide groups on the polyacrylamide to carboxylate groups. A limitednumber of the beta-alanine-type constituents add on to one or more ofthe amide groups on the PHPA to produce relatively low molecular weightbeta-alanine-type branches on the PHPA.

The beta-alanine-type branched PHPA of the present inventionsubstantially increases the viscosity of an aqueous fluid when addedthereto without increasing the potential for face plugging in theformation. The branched PHPA is also significantly more brine tolerantthan a corresponding straight-chain PHPA.

The benefits are believed attributable to the following phenomena.Generally, the viscosity enhancing capability of commonly usedstraight-chain PHPA is a function of its molecular weight, i.e., theviscosity of a polymer solution increases as the molecular weight of thepolymer dissolved therein increases. In contrast, the viscosityenhancing properties of the present branched polymer are a direct resultof branching rather than molecular weight. By placing a limited numberof branches on a PHPA molecule, the same degree of viscosity enhancementis achieved with a lower molecular weight branched PHPA than with ahigher molecular weight straight-chain PHPA. Furthermore, because of itslower molecular weight, the branched polymer is substantially lesslikely to plug the matrix pores and wellbore face than the highermolecular weight straight-chain polymer. Finally, the branched polymeris significantly more brine tolerant than the straight-chain polymerbecause the branching apparently minimizes balling up of the polymer onitself in the presence of a brine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a brine tolerant polymer compositionand a method of making and using the same in oil recovery applications.The polymer composition is a partially hydrolyzed polyacrylamide havingbranched beta-alanine-type constitutents. The polymer molecule has thefollowing formula: ##STR1## where: x is the fraction of amide groups inthe molecule and has a value between about 0.9 and 0.5;

y is the fraction of carboxylate groups in the molecule and has a valuebetween about 0.1 and 0.5 which represents the degree of hydrolysis ofthe PHPA molecule;

z is the fraction of branched groups containing the beta-alanine-typeconstituents in the molecule and has a value greater than 0 and lessthan about 0.01;

R₁ and R₂ are either repeating amide, carboxylate, or branched groups inthe stated fractional proportions or a chain terminating hydrogen ion;

M⁺ is the cation of the hydrolyzing salt, for example, NH₄ ⁺, K⁺, Na⁺,etc; and

R₃ is NH₂, O⁻ M⁺, or a repeating beta-alanine-type constituent.

It is preferable that the polymer has at least one branched betaalanineconstituent along the chain to increase the viscosity of a fluidcontaining the polymer. It is further preferable that the number ofbranched groups in the molecule does not exceed 1% of the total numberof groups making up the polymer molecule to avoid polymer plugging ofthe formation. The degree of hydrolysis of the polymer is about 10 to50% and preferably about 25 to 35%. The average molecular weight rangeof the polymer is about 2 million to about 10 million and preferablyabout 4 million to about 6 million. The molecular weight of thebranching constituents on the polymer is only a small portion of thetotal molecular weight of the polymer. Each branch usually has amolecular weight on the order of about 50 to about 300 although largerbranches may be possible with significant repetition of thebeta-alanine-type group.

Several alternatives are available for producing the branched PHPA. Inthe preferred method, a polyacrylamide is formed in an aqueous solutionby polymerizing an acrylamide monomer substantially to completionaccording to methods known in the art. The resulting polyacrylamidesolution generally has a concentration from about 3 to about 10% byweight. The polyacrylamide solution may be diluted to a concentrationless than 3% by weight to facilitate the subsequent process steps.

Thereafter an amount of acrylamide monomer and a base such as sodiumhydroxide are added to the polyacrylamide solution. The weight ratio ofpolymer to monomer in the resulting solution is from about 20:1 to about6:1 and preferably about 13:1 to about 10:1. A portion of the basereacts with the polyacrylamide, hydrolyzing 10 to 50% of the amidegroups. The remainder of the base reacts with the monomer, formingbeta-alanine-type constituents. The general formula of thebeta-alanine-type constituents is: ##STR2## wherein X₁ is NH₂, OH, or anamide group and X₂ is NH₂ or O⁻ M⁺. The beta-alanine-type constituentmay also be present in solution in polymerized form, wherein the generalformula of the constituent is repeated one or more times.

The beta-alanine-type constituents add onto one or more of the amidegroups of the PHPA resulting in the branched PHPA. The remainder ofbeta-alanine-type constituents are left in solution.

The hydrolysis and addition reactions are carried out at a temperaturerange from ambient to about 71° C. and preferably in a narrow rangearound 55° C.

This method is preferred because it is relatively easy to control theamount of monomer present in solution which subsequently determines thecritical degree of beta-alanine branching on the PHPA molecule.

In an alternative embodiment, the branched polymer is produced bypartially polymerizing an acrylamide monomer. The polymerizationreaction is terminated before the monomer reacts substantially tocompletion such that the weight ratio of polymer to excess monomerremaining in the polyacrylamide solution is from about 20:1 to 6:1 andpreferably about 12:1 to 8:1. Partial polymerization can be achieved bylimiting the amount of catalyst to the polymerization reaction, adding afree radical inhibitor to the reaction solution at an appropriate stageof the reaction, or cooling the reaction at an appropriate stage.Thereafter a base is added to the solution to partially hydrolyze thepolyacrylamide and initiate beta-alanine-type branching of the PHPA.

In another embodiment, PHPA is produced according to methods known inthe art and a base and monomer are added to the PHPA solution resultingin branched PHPA. In yet another method, a pre-manufacturedbeta-alanine-type constituent and a base are added directly to apolyacrylamide or PHPA solution to produce the branched PHPA.

The beta-alanine-type branched PHPA solution produced according to oneof the above-described embodiments is diluted with an aqueous diluent toa desired concentration which is determined by its anticipated use. Forexample, the PHPA solution may be diluted to 1000 ppm with a fieldproduced brine for injection into a subterranean hydrocarbon-bearingformation as an oil displacement fluid. Alternatively, the solution maybe diluted for use as a mobility buffer in a micellar flood.

The aqueous solution containing the beta-alanine-type branched PHPA ismore brine tolerant than straight-chain PHPA. Brine tolerance is definedas the ability of a viscous polymer solution to remain at or near itsoriginal viscosity when contacting a brine solution.

It is believed that the viscosity enhancing character of high molecularweight straight-chain PHPA is produced by intermolecular entangling ofthe long polymer chains. However, the ions in a brine cause astraight-chain PHPA molecule to ball up on itself. This reduces theviscosity of a PHPA solution because the balled up molecules are lesscapable of intermolecular entanglement. A limited number ofbeta-alanine-type branches on the PHPA chain maintains or even improvesthe viscosity enhancing character of PHPA by hindering intramolecularballing while facilitating intermolecular entangling. The degree ofbranching is limited to the preferred range because too much branchingcauses the polymer to plug the wellbore face and/or the matrix poreswhen injected into a formation.

The beneficial effect of beta-alanine-type branching is more pronouncedwhen applied to lower molecular weight PHPA than higher molecular weightPHPA because lower molecular weight PHPA has a higher proportion ofbranches relative to the overall polymer size. A beneficial improvementin brine tolerance may also be realized from the presence of residualbeta-alanine-type constituents in the polymer solution independent oftheir branching function. Beta-alanine-type constituents in abrine/polymer solution may associate with a portion of the cations inthe brine resulting in fewer cations available to react with and ball upthe polymer molecules.

The following examples illustrate the composition of the presentinvention and its method of manufacture. The examples are not to beconstrued as limiting the scope of the invention.

EXAMPLE 1

A 6% by weight polyacrylamide solution is prepared having a residualmonomer concentration of 700 ppm. The molecular weight of the polymer is5 million. The solution is diluted with fresh water to a polymerconcentration of 1% by weight. Thereafter the polymer solution isdivided into a series of samples and differing weight ratios ofacrylamide monomer are added thereto. Sufficient sodium hydroxide isalso added to each sample to hydrolyze 30% of the acrylamide groups onthe polymer. The samples are reacted to completion at 55° C. The samplesare then diluted with an NaC1 solution to a polymer concentration of 500ppm and an NaC1 concentration of 500 ppm. The viscosity of each dilutesample is measured at 22° C. The results are summarized below in Table1.

                  TABLE 1                                                         ______________________________________                                        Weight Ratio of Polymer to Monomer                                                                   Viscosity                                              in Initial Sample      (centipoise)                                           ______________________________________                                        86:1 (no additional monomer)                                                                         30                                                     Greater than 12:1      30                                                     Equal to or less than 12:1                                                                           34                                                     ______________________________________                                    

The results indicate that viscosity improvement caused by the formationof branched PHPA in a brine is a discontinuous step function of theweight ratio of polymer to monomer. No benefit is realized in allsamples wherein the weight ratio of polymer to monomer is greater than12:1. An approximate 13% improvement in viscosity is observed where thepolymer to monomer ratio in the sample is 12:1 or less. However, littleimprovement is observed by decreasing the polymer to monomer ratiosubstantially below 12:1 in the samples. Therefore, the optimum ratio ofpolymer to monomer appears to be around 12:1 for the present reactionconditions.

EXAMPLE 2

A 7.5% by weight polyacrylamide solution is prepared, having a residualmonomer concentration of 900 ppm. The molecular weight of the polymer is10 million. The solution is diluted with fresh water to a polymerconcentration of 1% by weight. Thereafter the polymer solution isdivided into a series of samples and treated in the same manner asExample 1. The results are summarized in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Weight Ratio of Polymer to Monomer                                                                   Viscosity                                              in Initial Sample      (centipoise)                                           ______________________________________                                        83:1 (no monomer added)                                                                              30                                                     Greater than 11:1      30                                                     Equal to or less than 11:1                                                                           35                                                     ______________________________________                                    

The same discontinuous step function of Example 1 which relates thepolymer to monomer weight ratio to viscosity applies within experimentalerror to the results of Example 2, although a higher molecular weightpolymer is being used.

EXAMPLE 3

Samples of polymer solutions are prepared and treated in the manner ofExample 1 wherein the molecular weight of the polymer is 5 millionexcept that the samples have a polymer concentration of 1000 pmm and anNaCl concentration of 20,000 ppm. The results are summarized in Table 3below.

                  TABLE 3                                                         ______________________________________                                        Weight Ratio of Polymer to Monomer                                                                   Viscosity                                              in Initial Sample      (centipoise)                                           ______________________________________                                        Greater than 12:1      6.9                                                    Equal to or less than 12:1                                                                           13                                                     ______________________________________                                    

The data of Table 3 indicate that the polymer solution with optimalbranching is substantially more brine tolerant than the untreated orinsufficiently branched polymer solution.

EXAMPLE 4

Samples of polymer solutions are prepared and treated in the manner ofExample 3 except that the molecular weight of the polymer is 10 million.The results are summarized in Table 4 below.

                  TABLE 4                                                         ______________________________________                                        Weight Ratio of Polymer to Monomer                                                                   Viscosity                                              in Initial Sample      (centipoise)                                           ______________________________________                                        Greater than 11:1      11                                                     Equal to or less than 11:1                                                                           14                                                     ______________________________________                                    

Comparing the data of Table 3 and Table 4, it appears that the branchingis more effective in improving the brine tolerance of a lower molecularweight than a higher molecular weight polymer. This supports the beliefthat higher molecular weight polymers do not benefit as much frombranching as low molecular weight polymers because the high molecularweight polymers have more opportunities to entangle even with branchingdue to their extended length.

While the foregoing embodiments of the invention have been described andshown, it is understood that the alternatives and modifications, such asthose suggested and others, may be made thereto and follow in the scopeof the invention.

I claim:
 1. A process for increasing the viscosity of an aqueouspartially hydrolyzed polyacrylamide solution and injecting said solutionhaving increased viscosity into a subterranean hydrocarbon-bearingformation to facilitate the recovery of hydrocarbons therefromcomprising:(a) providing said solution with a branched polymer in anamount sufficient to increase the viscosity of said solution whereinsaid branched polymer has the general formula: ##STR3## where: x is afraction of amide groups in the branched polymer and has a value betweenabout 0.9 and 0.5;y is a fraction of carboxylate groups in the branchedpolymer and has a value between about 0.1 and 0.5; z is a fraction ofbranched groups in the branched polymer and has a value greater than 0and less than about 0.01; R₁ and R₂ are either repeating amide,carboxylate, or branched groups in the stated fractions of x, y and z ora chain terminating hydrogen ion; R₃ is a branching constituent selectedfrom the group consisting of ##STR4## and dimers and trimers thereof;and M⁺ is a cation of a hydrolyzing salt; (b) injecting said solutioncontaining said branched polymer and said partially hydrolyzedpolyacrylamide into a wellbore in fluid communication with saidsubterranean hydrocarbon-bearing formation; and (c) displacing saidsolution into said formation to facilitate recovery of hydrocarbonstherefrom.
 2. The process of claim 1 wherein said solution is an oildisplacement fluid.
 3. The process of claim 1 wherein said solution is amobility buffer.
 4. The process of claim 1 wherein said solution is abrine.
 5. The process of claim 1 wherein M⁺ is selected from the groupconsisting of Na⁺, K⁺ and NH₄ ⁺.
 6. The process of claim 1 wherein y isabout 0.25 to about 0.35 and x is about 0.65 to about 0.75.
 7. Theprocess of claim 1 wherein said solution is a permeability reducingagent for conformance treatment.
 8. The process of claim 1 wherein saidpartially hydrolyzed polyacrylamide solution is provided with saidbranched polymer by adding a branching compound, having the generalformula: ##STR5## where: X₁ is selected from the group consisting ofNH₂, OH, and an amide group, andX₂ is selected from the group consistingof NH₂ and O⁻ M⁺,to said solution and reacting said partially hydrolyzedpolyacrylamide with said brancing compound in said solution to form saidbranched polymer.